1. Field of the Invention
This invention relates broadly to a chemical vapor deposition system. More particularly, this invention relates to the injectors and the relationship between the injectors and the nozzle in a plasma jet chemical vapor deposition system for producing diamond films.
2. State of the Art
The utility for high quality thin diamond films for various applications is well known. Superior physical, chemical, and electrical properties make diamond films desirable for many mechanical, thermal, optical and electronic applications. For example, diamond has the highest room-temperature thermal conductivity of any material, a high electric field breakdown (.about.10.sup.7 V/cm), and an air stable negative electron affinity. These properties make possible high power, high frequency transistors and cold cathodes, which cannot be made with any semiconductor other than diamond.
One method for producing thin diamond films is by using a chemical vapor deposition (hereinafter `CVD`) system. In CVD systems, a mixture of hydrogen and a gaseous hydrocarbon, such as methane, is activated and contacted with a substrate to produce a diamond film on the substrate. The hydrogen gas is disassociated into atomic hydrogen, which is then reacted with the hydrocarbon to form condensable carbon radicals including elemental carbon. The carbon radicals are then deposited on a substrate to form a diamond film. Some CVD methods use a hot filament, typically at temperatures up to 2000.degree. C., to provide the thermal activation temperatures necessary to bring about the conversion described above. Other CVD methods use a plasma jet.
In some plasma jet methods, hydrogen gas is introduced into a plasma torch which produces a hydrogen plasma jet by means of a direct current arc (hereinafter "DC arc"), or an alternating current arc ("AC arc"), or microwave energy. The plasma torch is hot enough to reduce gases to their elemental form. However, the energy level of the plasma jet has a tendency to fluctuate. One method of stabilizing the energy level of the plasma is to utilize a vortex design in the CVD system. Tangential injection of the hydrogen gas into the arc processor may be used to impart the vortex to the hydrogen, in gaseous and atomic form.
The vortex design results in a controlled swirl of plasma. Hydrogen gas is introduced into the primary jet and some of the hydrogen gas is thereby disassociated into monatomic hydrogen. The hydrogen (in both elemental and molecular states), swirling according to the swirl of the plasma, is forced through a downstream injector system which introduces jets of hydrocarbon needed to react with the elemental hydrogen to form diamond films.
Referring to FIG. 1, a prior art DC arc plasma jet system 10 is shown. The assembly includes a hydrogen gas inlet 12, a cathode 14, a cylindrical chamber 16 having cylindrical walls 17, an anode 18, downstream injectant ports 20a, 20b, a gas injection disc 22 having a plurality of radially-positioned injectors 24a-24h (shown only with respect to 24a and 24b for purposes of clarity), and a nozzle 26 directed toward the substrate. The hydrogen gas enters the hydrogen gas inlet 12 and is heated to a partial plasma state by an arc across the cathode 14 and the anode 18. The arc is controlled by solenoids (not shown) surrounding the chamber. The tangential injection of the hydrogen contains the plasma and imparts the vortex swirl to the plasma. Downstream, hydrocarbon injectant and carrier hydrogen gas enter through the injectant ports 20a, 20b into the gas injection disc 22, and out of the injectors 24a, 24b where the injectant mixes and reacts with the hydrogen plasma, resulting in a mixture of molecular hydrogen, atomic hydrogen and carbon radicals which exits through the nozzle 26.
FIG. 2 illustrates the gas injection disc 22 provided with radially aligned injectors 24a-24h. Each injector is aligned along a radius formed from the periphery of the ring to the center of the ring. Referring to FIG. 3, with reference to one injector 24a, the injector is a substantially cylindrical bore having three portions: a relatively large diameter inlet hole 30, a tapered frustoconical portion 32, and a relatively small diameter outlet hole 34.
There are several known problems associated with state of the art plasma jet systems. For example, various challenges and problems have been encountered with the hydrocarbon injectors. With reference to FIG. 3, a first shortcoming of the prior art injectors is that when using high enthalpy, high energy rate recipes, the injector outlet holes clog. One potential cause of the clogging is that the expanding injected jets entrain atomic hydrogen from the primary jet, bringing atomic hydrogen into the injectors and forming diamond or diamond-like carbon deposits at the outlet hole.
Another problem associated with the injectors of a plasma jet system is that when using a vortex stabilized arc engine, the vortex has a detrimental effect on downstream mixing of the injected hydrocarbon gas. The outer swirl of the vortex consists mainly of molecular hydrogen in equilibrium between the centrifugal force of the swirl and the resultant static pressure gradient throughout the jet. The atomic hydrogen, with half the mass of molecular hydrogen, is drawn towards the center of the swirling jet. Radially injected hydrocarbon, being many times heavier than molecular hydrogen, is forced to the outermost portion of the swirling jet, resulting in a non-uniform mixture of hydrogen and hydrocarbons. Consequently, a diamond film produced from a non-uniform mixture may have a slow growth rate and poor quality.
An additional problem with the use of high enthalpy, high energy rate recipes is that the injectors are subject to excessive heating and are subject to high thermal gradients caused by non-uniform cooling of the injectors at shutdown. The excessive heating and thermal gradient cause the injectors to crack and may further contribute to injector hole clogging. Referring back to FIGS. 1 and 2, two possible causes for the excessive heating of the injectors 24a-24h are direct impingement of recirculated arc gas on the bottom face 22a of the gas injection disc 22, and conduction from the nozzle 26 which is heated by the recirculated gas.
Furthermore, the mixing of downstream injected hydrocarbon gas with a primary flow of swirling hydrogen is a key consideration affecting film growth rate. The flow of hydrocarbon injectant out of the injector is often optimized for mixing with a given flow of hydrogen only by trial and error. However, once a specific injector has been found to give optimum results for a given primary flow of hydrogen it would be desirable to be able to design a hydrocarbon injector which provides the same level of mixing at a different flow rate of injectant through the injector without entering into a new trial and error process.